Portable electronic devices frequently run off of batteries. Data delivered to the devices is often short (e.g. email, instant messaging, numerical updates, status changes). The power dissipated by the devices while looking for transmissions that may be directed to them can often exceed the power required to receive such transmissions. Moreover, the devices typically only occasionally have transmissions directed to them. Despite the fact that most devices only occasionally have transmissions directed to them, the devices must nevertheless stay awake for the entire notification period. This means that many devices are dissipating power during the entire notification period, even though usually there is no data for them.
It would therefore be desirable for devices to be able to quickly determine if data is being sent to them, and if not return to a sleep mode so as to conserve energy.